A differential is used to distribute torque between two interconnected rotating shafts. A specific, everyday use, of a differential is on a motor vehicle where a left and a right axle of the vehicle are rotatably connected via a differential.
When a vehicle turns a corner the outer wheels of the vehicle must travel further than the wheels on the inner side of the vehicle, as the turning arc is longer farthest from a turning centre. To accommodate this turning requirement, the outer wheels of a vehicle are accelerated proportionally to the inner wheels being decelerated, thereby allowing the vehicle to turn a corner whilst maintaining all four wheels in rolling contact with a road surface. Without a differential either the outer wheels will travel too slowly or the inner wheels will travel too fast, either or which will result in poor, unpredictable handling characteristics of the vehicle, loss of traction, and tyre damage.
The differential is the gearing unit that proportionally increases the speed of the outer wheel and decreases the speed of the inner wheel by equally distributing torque simultaneously between the axle of the outer wheel and the axle of the inner wheel.
Vehicle differentials are found between the rear wheels of a rear wheel drive vehicle, effectively splitting a single rear axle into left and right rear axles, respectively coupled to the left and right rear wheels. Differentials can also be disposed between the front wheels of a vehicle often in conjunction with a gearbox as a “transaxle”, and in some cases intermediary to a first and a second differential, such as a transfer box.
The differential is typically driven directly, or indirectly, by the prop shaft of the vehicle and utilises a hypoid gear pair. The first gear of the hypoid pair is a pinion gear connected to the prop shaft and the second gear of the hypoid pair is a ring gear or crown gear mounted to a carrier of the differential gear set.
Although a road vehicle relies on the differential to maintain good handling and control for on road driving, there are times when a vehicle is driven off-road or when the road conditions become poor, such that the equalising of torque between the two interconnected wheels is not wanted.
For example, when the road becomes icy one or both the inner and outer wheel of a vehicle may lose traction (or grip). However, when only one wheel loses traction, the vehicle may become stuck. An open differential distributes torque equally to the inner and the outer wheel. The torque is applied until the wheel exceeds a maximum traction, the traction being a factor of the downforce on the wheel and the frictional coefficient of the surface upon which the wheel is contacting. If one wheel is sitting on ice, the friction of the surface is very low, thus significantly dropping the traction of the wheel, accordingly, very little torque is required to make the wheel spin. As the differential splits torque evenly, the amount of torque distributed to both wheels is limited by the traction threshold of the ice-bound spinning wheel. As such, both the wheels connected to the differential are significantly limited in power, thus the vehicle becomes stuck until the friction under the spinning wheel is increased.
The above, loss of traction situation, is overcome by having a vehicle with a straight axle joining the left and right wheels. As there is no equalising of torque between the two wheels, loss of traction on one wheel will redistribute the torque delivered to the adjacent wheel. However, a straight axle (beam axle or dead axle) will not be capable of turning a corner without dragging the wheels of the vehicle. Furthermore, a straight axle cannot maintain even road contact over uneven road surfaces. The same effect is achieved by closing or locking a differential, so that the left and right axles are locked together and turn synchronously.
In light of the above there are a range of available differentials, some with complex clutch systems that “limit” the distribution of torque in a wheel spin situation, so that torque is redirected to the non-spinning wheel thereby assisting the vehicle to free itself. These are often referred to as limited-slip differentials. These are primarily “open” differentials with the capability of allowing the left and right axle to vary in angular velocity to one another and limit the extent of the variance ie they lock or “close” the differential when the difference in angular velocity between the two axles exceeds a limit or threshold.
Some differentials are configured to be selectively activated ie turned on or off on demand. These are referred to as “lockers”. Lockers; therefore, provide either an open differential or a closed differential. They can be manually actuated by a switch or alternatively, they can use complex internal electrical and mechanical systems to assess the terrain and assess when the differential requires locking.
In light of the above, it is always desirable to provide a new differential to address or ameliorate the limitations described above or to at least provide a useful alternative.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, a limited number of the exemplary methods and materials are described herein.